1. Field of the Art
The present invention relates to a process of producing multiple-layer filter medium consisting of a porous support body and at least one filtering layer which is formed on the support body and which has minute pores.
2. Related Art Statement
There is known a multiple-layer filter medium which consists of a porous support body of tubular or other shapes formed of a metallic or ceramic material and having a high mechanical strength, and at least one filtering layer formed of a material similar to that of the porous support body and having minute pores. The filtering layer or layers is/are formed integrally on one of opposite surfaces of the porous support body (on the inner or outer surface of the tubular support body). This type of filter medium has a relatively low resistance to a flow of a liquid to be filtered, and is very much effective to separate fine solid particles from a large amount of flow of the liquid. The filter medium having a tubular porous support body is produced, for example, by a process disclosed in Japanese Patent Application published in 1981 (for opposition puprpose) under Publication No. 56-8643. This process includes the steps of: preparing a slurry for forming the filtering layer; introducing the slurry into the tubular porous support body; rotating the support body so that the slurry is spread over the inner surface of the support body, by a centrigufal force, while at the same time holding the outer surface of the support body under a pressure which is lower than that acting on the inner surface, so that a layer of the slurry tightly adheres to the inner surface of the support body; and firing the porous support body with the layer of the slurry on its inner surface. An alternative process is known according to U.S. Pat. No. 4,356,215. This process includes the steps of: preparing two masses of coating suspension which contain mineral particles having different sizes, respectively; applying one of these two masses of coating suspension to a surface of the porous support body, to form a first coating layer, heating and drying the first coating layer, and subsequently firing the porous support body with the first coating layer; and applying the other masses of coating suspension to the fired first coating layer to form a second coating layer, heating and drying the second coating layer, and finally firing the porous support with the first and second coating layers.